1. Field of the Invention
The present invention relates to a crystallization apparatus and method which fuse and crystallize a crystallization target area such as a polycrystal semiconductor film of a processed substrate by using light rays, a manufacturing method of an electronic device and an electronic device.
2. Description of the Related Art
For example, as a method for crystallizing an amorphous semiconductor, e.g., an amorphous silicon thin film formed on a glass substrate of a liquid crystal or electroluminescent (EL) display apparatus without giving heat damages to this glass substrate, an excimer laser crystallization method has been developed. This technique homogenizes an intensity of a light irradiation cross section of excimer light by using a homogenizing optical system, forms the excimer light into a rectangular shape (e.g., a cross-sectional shape is 150 mm×200 μm) through a metallic mask having an elongated rectangular opening, and projects the obtained excimer light. Scanning is effected by linearly relatively moving in one direction a surface of an amorphous silicon thin film previously deposited on a glass substrate by using the projected laser light, and the laser light is intermittently applied in a short axis direction at intervals of 10 μm.
As a result, the amorphous silicon thin film which absorbed this applied laser light is fused, its temperature is lowered when the incidence of the laser light is interrupted, and the amorphous silicon is converted into polycrystal silicon. In this technique, even if a substrate formed of a material such as general glass or plastic which has a weakness for intense heat is used, heat damage is not generated in the substrate. That is because the excimer laser is a pulse laser which is approximately 20 nm, and a crystallization process period is completed in approximately 50 to 100 ns. A crystal particle size of polycrystal crystallized in this manner is dependent on a laser energy density, and the particle or grain size is approximately 0.1 to 1 μm, thereby forming a polycrystal silicon thin film formed of crystal grains having such a particle size.
As a technique developed from the excimer laser annealing, there is known a technique called a sequential lateral solidification (SLS) mode (see, e.g., Journal of the Surface Science Society of Japan, Vol. 21, No. 5, pp. 278-287, 2000). According to this technique, excimer laser light whose light intensity is homogenized by a homogenizing optical system is passed through a mask to which a metal narrow slit which is approximately 2 microns is provided, thereby forming its cross section into a rectangular shape.
When a fluence (energy density) of the laser light which has passed through the narrow slit is set in such a manner that the amorphous silicon thin film is fully fused in a thickness direction, lateral growth occurs from an outer area of the narrow slit toward the inner side, thereby forming crystallized silicon. Then, when a sample is moved in one direction by two microns and laser light is applied thereto, the fused silicon grows in the lateral direction with one end portion of the crystallized silicon formed by the laser application being used as a seed crystal. Repeating the laser application and the sample movement process can form the polycrystal silicon thin film with a large particle size.
As a method obtained by further developing the excimer laser crystallization method, there is known a phase modulation excimer laser crystallization method (e.g., PCT National Publication No. 2000-505241). Characteristics of this method lie in that excimer laser light is modulated to have a laser light intensity distribution having an inverse peak pattern in which a minimal light intensity portion corresponds to a phase shift portion by transmitting the excimer laser light through an optical component called a phase shifter (e.g., one having a linear phase shift portion formed between two types of areas whose phases are shifted by 180 degrees by performing step machining with respect to a quarts plate and forming these two types of areas). Pulse application is carried out with respect to, e.g., an amorphous silicon thin film formed on a substrate by using the thus modulated laser light, and an irradiation area is crystallized in accordance with each irradiation.
As different from the excimer laser crystallization method and the SLS mode, this method does not use a homogenized light intensity distribution, and the same area does not have to be irradiated with the laser light for many times. In this method, a temperature distribution which is inclined in accordance with an inverse pattern is generated in the amorphous silicon thin film irradiated with the laser light due to the modulated light intensity distribution having the inverse pattern, and a crystal nucleus, i.e., a crystal seed is formed at a position where the energy is small. Further, a growth distance is increased due to lateral growth based on this crystal nucleus, thereby obtaining a crystal grain with a large particle size. A crystal grain with a large particle size can, therefore, be formed while controlling a position of the crystal grain.
In regard to the excimer laser crystallization method explained first, a crystal grain size is approximately 1 to 2 microns at the maximum level and approximately 0.05 micron at the minimum level, and a crystal particle size is intensely dependent on a fluence of the laser light. Therefore, the crystal particle size becomes uneven unless a laser light intensity is homogenized. As a result, irregularities are generated in transistor characteristics (a threshold voltage, a subshred coefficient, a mobility). In general, a channel area of an MOS transistor which requires a length of not less than 4 μm cannot be formed in only one crystal grain based on a crystal particle size which is approximately 1 to 2 microns at the maximum level, and it must be formed across a plurality of crystal grains. Therefore, there is a problem that a plurality of crystal grain boundaries are formed in each channel area and differences in crystal grain boundary number results in differences in characteristics, thereby forming respective transistors. Furthermore, when electrons (holes) move across a crystal grain boundary, the crystal grain boundary becomes an obstacle, which affects the mobility.
In regard to the SLS mode described after the excimer laser crystallization method, since nearly a half of the laser light is shielded by using a metallic mask, the laser energy cannot be effectively utilized. Moreover, since crystallization is a carried out with a crystal particle size of 1 μm or below, there is a problem that irregularities are generated in transistor characteristics (a threshold voltage, a subshred coefficient, a mobility) like the first prior art.
In regard to the phase modulation excimer laser crystallization technique described at-last, this is a technique which can perform extensive crystallization with a crystal particle size of, e.g., approximately 6 μm or above, and is an excellent crystallization technique which can manufacture a channel area of a transistor in one crystal grain boundary. The present inventors and others has developed an industrialization technology of a high-performance display apparatus utilizing this crystallization technique. In this process, there is a demand to accurately manufacture a transistor in a crystallized area in a unit of μm so that at least one channel area having a length of not less than 4 μm can be positioned in each crystal grain. Even if a crystal grain with a large particle size can be formed, a channel area cannot be formed in a crystal grain in an exposure step in particular when the positioning accuracy of the channel area of each transistor and each crystal grain cannot be obtained. Each thin-film transistor having such a channel area cannot obtain desired even characteristics, e.g., a high electron mobility. For example, if a channel area of each pixel switching transistor of a display apparatus is formed by using the above-described technique, a response speed varies depending on each position, thereby resulting in display unevenness.